1. Field of the Invention
The present invention relates to optical integrators.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of a flat panel display (or other device). This pattern can be transferred onto all or part of the substrate (e.g., a glass plate), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate.
Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots. Instead of a mask, the patterning device can be a patterning array that comprises an array of individually controllable elements. The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. Lithographic apparatus designed to expose a substrate of this type can provide an exposure region that covers a full width of the rectangular substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, half of the width of the substrate, then the substrate can be moved transversely after the first scan, and a further scan is typically performed to expose the remainder of the substrate.
A lithographic apparatus includes an optical illumination system. It is frequently desirable in an optical illumination system to produce a substantially telecentric beam in which the intensity of the light, across a plane orthogonal to the optical axis (the axis that defines the direction of propagation of the beam), has a substantially uniform distribution. Unfortunately, because the illumination source usually produces a beam in which this intensity has a Gaussian distribution and/or a beam having a relatively low degree of telecentricity, an optical device is often disposed in the optical illumination system to modify the beam to compensate for this situation. In many optical illumination systems, this optical device is an optical integrator.
An optical integrator is an object having: (1) a surface is configured to surround the optical axis, or (2) two separate surfaces with the optical axis between them. A volume within the optical integrator can be an object made of a material that is transparent to a wavelength of light produced by the illumination source, a gas, or a vacuum. The optical illumination system is configured to cause the beam to enter the optical integrator so that the light reflects from the surface or surfaces so that the beam that exits the optical integrator has a more uniform intensity distribution and/or is more telecentric.
It is generally understood that the uniformity of the intensity distribution of a beam and the telecentricity of the beam increase with the number of reflections that the light experiences in an optical integrator. Typically, the number of reflections can be increased by: (1) increasing the length of the optical integrator along the optical axis, or (2) reducing the area of the optical integrator in the plane orthogonal to the optical axis and defined by the surface or surfaces of the optical integrator.
Increasing the length of the optical integrator entails an increase in the length of the optical illumination system, which increases the degree to which energy in the beam is absorbed. Reducing the area of the optical integrator in the plane orthogonal to the optical axis and defined by the surface or surfaces of the optical integrator entails an increase in the magnification power of the relay lens that is usually disposed at the exit of the optical integrator. Both approaches increase the cost and complexity of the optical illumination system. What is needed is a relatively short optical integrator with a surface or surfaces configured to define a relatively large area in the plane orthogonal to the optical axis, wherein the optical integrator is configured to cause a relatively high number of reflections from this surface or surfaces.